With advances in technology, gas discharge laser light sources operate reliably for several billions of pulses. However, as the laser system operates, the gas gain medium, typically a mixture of a halogen gas, a noble gas, and a buffer gas (e.g., fluorine, argon, and neon, or other known gases), becomes depleted of the halogen gas and accumulates debris. This reduction in the halogen gas degrades gain in the discharge medium. To compensate, discharge voltage is increased over the life of the gas to maintain a constant output pulse energy level.
In addition, if the debris is not effectively removed from the discharge area (by, e.g., gas circulation around the chamber) between subsequent discharges, a phenomenon known as arcing can occur. Arcing results in non-uniform discharges. Arcing also reduces the energy delivered to the gas gain medium for causing lasing and thus reduces and destabilizes the output pulse energy. This is a problem because the level and stability of output pulse energy are crucial in many laser applications, especially so in integrated circuit photolithography where arcing can negatively affect device yield and performance.
Other factors can also affect output pulse energy such as electrode erosion, which can both affect the discharge and create more debris.
These issues and the basic operation and control of a laser light source, including the relationship between electrode voltage control and output pulse energy, are known in the art (see, e.g., commonly-assigned U.S. Pat. No. 5,887,014, entitled PROCESS FOR SELECTING OPERATING RANGE FOR NARROW BAND EXCIMER LASER, issued to Das on Mar. 23, 1999). The '014 patent in turn incorporates the teachings of commonly-assigned U.S. Pat. No. 4,959,840, entitled COMPACT EXCIMER LASER INCLUDING AN ELECTRODE MOUNTED IN INSULATING RELATIONSHIP TO WALL OF THE LASER, issued to Akins et al. on Sep. 25, 1990.
The earlier '840 patent describes a halogen gas photolithography deep ultraviolet (DUV) laser light source having a “pair of spaced electrodes [that] are located within the laser cavity and form an electrical discharge area between the electrodes for stimulating gas within the discharge area to lasing action in accordance with an electrical discharge between the electrodes.” ('840 patent, Abstract) The '840 patent teaching that such DUV laser light sources can control laser output pulse energy by controlling the voltage applied to the electrodes was elaborated upon by the '014 patent teaching that such voltage control is at least one form of output pulse energy control:                “[i]t is known that within the normal operating range of the KrF laser, output pulse energy can be increased by increasing the pulse discharge voltage; and it can be increased by increasing the fluorine concentration. Increases or decreases in both or either of these parameters is easily accomplished with these narrow band KrF excimer lasers.”('014 patent, Col. 1, lines 32-37)        
The production and control of the voltage delivered to the electrodes of such a DUV laser photolithography light source using a step-up transformer is described in U.S. Pat. No. 5,936,988, entitled HIGH PULSE RATE PULSE POWER SYSTEM, issued to Partlo, et al. on Aug. 10, 1999. The '988 patent discloses a high repetition rate, high voltage power supply for a DUV laser photolithography light system operating at about 2000 pulses per second, and therefore, creating discharges between the electrodes at that same rate. ('988 patent, Abstract)
The '988 patent also discloses a pulse energy controller that uses a photodiode to monitor output pulse by comparing a current energy signal with historical pulse energy data to set a command voltage for the next pulse. ('988 patent, Col. 4, line 43-Col. 5, line 17)
The '014 patent also teaches maintaining constant output light pulse energy in the face of changing fluorine concentration in the laser gas. In particular, the '014 patent states that:                “[a] typical operating plan for producing constant laser pulses is to compensate for the fluorine depletion by increases in the discharge voltage. This is accomplished with a feedback control which monitors pulse energy on a ‘per pulse’ basis at pulse frequencies such as 1,000 Hz and controls the voltage to maintain substantially constant pulse energy as the fluorine concentration decreases over time. Normally the operating plan will encompass a voltage control range so that when the voltage increases to compensate for the depleted fluorine, reaches an ‘upper limit’ (usually requiring a period of about two hours), a quantity of fluorine is injected during a period of a few seconds. The quantity injected is predetermined to correspond to roughly to the quantity which would have been depleted over the two hour period. During the fluorine injection period, the automatic feed back control will force the voltage down in order to keep pulse energy substantially constant so that at the end of the injection period the voltage is approximately at the low level of the voltage operating range and fluorine pressure is approximately at its high level.”('014 patent, Col. 1, lines 44-63)        
The '014 patent also recognizes that these issues ultimately affect the continued use of a laser system by stating that “the operating life of the laser is adversely affected by increased fluorine concentration and also by increased discharge voltage.” ('014 patent, Col. 2, lines 42-44).
Demands for better response to fluorine consumption and its affect on laser operations, laser gas life (time between gas refills) and laser chamber life (time to chamber replacement) have been addressed, at least in part, by improvements in laser control systems that deliver longer operating life until an “end of life” (“EOL”) condition occurs. Such laser control system improvements include better gas injection control, better management of debris over time, and better estimation of operating parameters and EOL prediction algorithms.
Despite these advances, the basic approach of using voltage control to maintain laser system output pulse energy in response to diminishing fluorine concentration and increasing debris in the chamber has not changed. As a result, current laser control systems still typically respond to diminishing fluorine concentration by increasing the discharge voltage.
However, as higher and higher discharge voltages are commanded, at some point the discharge voltage can exceed power supply design limits and capabilities. This can disturb pulse energy and pulse energy stability along with the occurrence of arcing through the combination of high debris in the gas and higher charging voltage. The laser controller treats any of the foregoing as EOL conditions that result in either 1) an error message being sent to a tool utilizing the laser (e.g., a scanner) or 2) laser system and/or tool shutdown for some maintenance procedure to be performed to eliminate the cause of the EOL condition, or both.
Typically the determination to issue an EOL alarm or shut down the laser, either of which cause an automatic or manual shutdown of a utilizing tool, such as a scanner, can be made by the laser system, tool or both. The determination may be based on one or more current operating conditions which the controller(s) evaluates based on historical data which indicate the existence of or a trend toward an out of specification condition, in order to prevent the out of specification condition from actually occurring.
The EOL condition may call for a single maintenance action, such as a laser chamber gas refill which requires shutting down the laser light source system, or the replacement of the chamber itself. Additionally the maintenance called for may require the replacement of other laser system modules, such as the optics module, power supply module, or the like. Previously, EOL conditions relating to the laser gas, laser chamber, or other laser subsystems have occurred at unanticipated times. Consequently, the utilizing tool is subjected to an unanticipated or unscheduled shutdown with consequent throughput losses.
What is therefore needed is an improved approach to dealing with end of life conditions that avoids such unscheduled shutdowns.